I see that it’s a (common) problem with Android > 8: how can get notifications? K-9 Mail doesn’t seem work in background, even after giving all permissions.
Telegram FOSS uses a workaround for this (quite insane) Google policy: a permanent “push service” (I don’t know how it works, but from Telegram I get notifications).
I tried to keep opened K-9 Mail and to send an e-mail: no notification even the app was opened. So the problem doesn’t seem to work in background, but a lack of notification.
There is some workaround in K-9 Mail to get notifications?
My version is 5.600, from F-Droid.